15067Csientific Theories to Meditate About

Could our universe be a membrane floating in
higher dimensional space, repeatedly smashing
into a neighbouring universe? According to an
offshoot of string theory called braneworld,
there are large extra dimensions of space, and
while gravity can reach out into them, we are
confined to our own "brane" universe with only
three dimensions. Neil Turok of Cambridge
University in the UK and Paul Steinhardt of
Princeton University in New Jersey, US, have
worked out how the big bang could have been
sparked when our universe clashed violently
with another. These clashes repeat, producing
a new big bang every now and then - so if the
cyclic universe model is right, the cosmos could
be immortal.

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2. Evolving universes

When matter is compressed to extreme densities at
the centre of a black hole, it might bounce back
and create a new baby universe. The laws of
physics in the offspring might differ slightly,
and at random, from the parent - so universes
might evolve, suggests Lee Smolin of the Perimeter
Institute in Waterloo, Canada. Universes that make
a lot of black holes have a lot of children, so
eventually they come to dominate the population of
the multiverse. If we live in a typical universe,
then it ought to have physical laws and constants
that optimise the production of black holes. It is
not yet known whether our universe fits the bill.

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3. Superfluid space-time

One of the most outlandish new theories of
cosmology is that space-time is actually a
superfluid substance, flowing with zero friction.
Then if the universe is rotating, superfluid
spacetime would be scattered with vortices,
according to physicists Pawel Mazur of the
University of South Carolina and George Chapline
at Lawrence Livermore lab in California  and
those vortices might have seeded structures such
as galaxies. Mazur suggests that our universe might
have been born in a collapsing star, where the
combination of stellar matter and superfluid space
could spawn dark energy, the repulsive force that
is accelerating the expansion of the universe.

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4. Goldilocks universe

Why does the universe have properties that are
"just right" to permit the emergence of life?
Tinker with a few physical constants and we would
end up with no stars, or no matter, or a universe
that lasts only for the blink of an eye. One answer
is the anthropic principle: the universe we see
has to be hospitable, or we would not be here to
observe it. Recently the idea has gained some
strength, because the theory of inflation suggests
that there may be an infinity of universes out
there, and string theory hints that they might
have an almost infinite range of different
properties and physical laws. But many cosmologists
dismiss the anthropic principle as being non-science,
because it makes no testable predictions.

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5. Gravity reaches out

Dark matter might not really be "stuff"  it
could just be a misleading name for the odd
behaviour of gravity. The theory called MOND
(modified Newtonian dynamics), suggests that
gravity does not fade away as quickly as current
theories predict. This stronger gravity can fill
the role of dark matter, holding together galaxies
and clusters that would otherwise fly apart. A new
formulation of MOND, consistent with relativity,
has rekindled interest in the idea, although it
may not fit the pattern of spots in the cosmic
microwave background.

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6. Cosmic ghost

Three mysteries of modern cosmology could be
wrapped up in one ghostly presence. After making
an adjustment to Einstein's general theory of
relativity, a team of physicists found a strange
substance popping out of their new theory, the
"ghost condensate". It can produce repulsive
gravity to drive cosmic inflation in the big bang,
while later on it could generate the more sedate
acceleration that is ascribed to dark energy.
Moreover, if this slippery substance clumps
together, it could form dark matter.

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7. It's a small universe

The pattern of spots in the cosmic microwave
background has a suspicious deficiency: there
are surprisingly few big spots. One possible
explanation is that the universe is small - so
small that, back when the microwave background
was being produced, it just could not hold those
big blobs. If so, space would have to wrap around
on itself somehow. Possibly the oddest suggestion
is that the universe is funnel-shaped, with one
narrow end and one flared end like the bell of a
trumpet. The bent-back curvature of space in this
model would also stretch out any smaller microwave
spots from round blobs into the little ellipses
that are indeed observed.

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8. Fast light

Why do opposite sides of the universe look the
same? It's a puzzle because the extremes of today's
visible universe should never have been in touch.
Even back in the early moments of the big bang,
when these areas were much closer together, there
wasn't enough time for light - or anything else -
to travel from one to another. There was no time
for temperature and density to get evened out; and
yet they are even. One solution: light used to move
much faster. But to make that work could mean a
radical overhaul of Einstein's theory of relativity.

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9. Sterile neutrinos

Dark matter might be made of the most elusive
particles ever imagined - sterile neutrinos.
They are hypothetical heavier cousins of ordinary
neutrinos and would interact with other matter
only through the force of gravity - making them
essentially impossible to detect. But they might
have the right properties to be "warm" dark matter,
buzzing about at speeds of a few kilometres per
second, forming the largish dark matter clumps
mapped by recent observations. Sterile neutrinos
could also help stars and black holes to form in
the early universe, and give the kicks that send
neutron stars speeding around our galaxy.

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10. In the Matrix

Maybe our universe isn't real. Yale Philosopher
Nick Bostrum has claimed that we are probably living
inside a computer simulation. Assuming it ever
becomes possible to simulate consciousness, then
presumably future civilisations would try it,
probably many times over. Most perceived universes
would be simulated ones - so chances are we are in
one of them. In that case, perhaps all those
cosmological oddities such as dark matter and dark
energy are simply patches, stuck on to cover up
early inconsistencies in our simulation.